Along with requirements to semiconductor devices to be compact and have high performance in recent years, various adhesives for electronic materials have been developed. These adhesives for electronic materials are required to be highly reliable and to secure it, epoxy resins scarcely shrinking by curing, having high adhesive capability, made various in types, and easy for planning for blending have most popularly been used. As such epoxy resins, for example, common liquid-state epoxy resins such as bisphenol A type liquid-state epoxy resins and bisphenol F type liquid-state epoxy resin have been generally often used because of excellent workability. However, adhesives for electronic materials containing such liquid-state epoxy resins for common use tend to be unsatisfactory to the requirements for rather high reliability of the present time and innovative and highly capable epoxy resins have been developed. The adhesives of the invention include pressure sensitive adhesives.
Practical properties presently required for the adhesives for electronic materials to have reliability are, for example, heat resistance, moisture resistance, resistance to thermal cycles, resistance to solder reflow, and the like and among them, with respect to the moisture resistance and resistance to solder reflow, it is required that cured adhesives indispensably have a low water absorption coefficient and low water absorption amount. This is because if the water absorption coefficient of a cured adhesive is high, water easily penetrates the adhesion interface and may possibly lowers the adhesive force in the interface. Also, if the water absorption amount of the cured adhesive is high, the water steeply evaporated at a solder reflow temperature, which may be as high as 200 to 260° C. to possibly break electronic parts.
On the other hand, in order to improve the resistance to thermal cycles, a large quantity of an inorganic filler is added generally to lower a linear expansion coefficient (linear expansion ratio). It is attributed to that the inorganic filler has a linear expansion coefficient much smaller than that of an organic filler. However, if a large quantity of the inorganic filler is added, although the linear expansion coefficient becomes low, the elastic modulus of an adhesive is increased and therefore, it leads to a problem that the cured adhesive becomes hard to relax the stress. That is, a method of improving the resistance to thermal cycles by adding the inorganic filler has limitations. Also, there is a problem that in the case of using the adhesive in form of an adhesive sheet obtained by processing the adhesive in sheet-like shape, the inorganic filler added thereto lowers the strength of the adhesive sheet before curing or makes processing by laser difficult at the time of using the adhesive sheet for a substrate required to have via holes or makes formation of via holes with a high precision difficult.
With respect to the improvement of resistance to thermal cycles, it is generally carried out to add a rubber polymer such as acrylic rubber in order to lower the stress to be generated (e.g. reference to Japanese Patent Publication No. 3,342,703)
However if the rubber polymer is added, although the resistance to thermal cycles is improved, the stress is relaxed by sacrificing the heat resistance and therefore, it is very difficult to satisfy high heat resistance and moisture resistance as well as the resistance to thermal cycles simultaneously. That is, to achieve the resistance to thermal cycles in advanced level, it is required to relax the stress to be generated at the time of thermal cycles.
To cause the effect to relax the stress, there is a method of adding a component for providing flexibility such as carboxylic acids; glycidyl-modified polyolefins; diene type rubber polymers having functional groups such as CTBN, ATBN, or the like; nitrile rubber; silicones having reactive groups in the terminals; acrylic rubber; or styrene type elastomers is added in a compatible or phase-separation manner. However, in the case where these flexibility-providing components are compatible with the epoxy resin to be a matrix resin, the heat resistance is considerably decreased and it becomes impossible to exhibit a high heat resistant adhesive capability at high temperature. Further, even in the case of these flexibility-providing components are in the phase-separation structure, the flexibility-providing components and the epoxy resin slightly become mutually compatible in their interface to result in tendency of heat resistance decrease. The phase-separation structure is not necessarily stable to temperature alteration and therefore, it may be possible to be compatible state depending on the temperature alteration.
Conventionally, an epoxy resin type curable composition often contains an acid anhydride and the like as a curing agent and in such a case, an un-reacted product remains in the cured product after curing in some cases. There is a problem that the un-reacted product become acidic or alkaline by reaction caused by moisture absorption and for that, an acidic substance or an alkaline substance flows out to the cured product surface and the periphery thereof and causes an adverse problem such as corrosion of an electrode metal such as aluminum and copper. Further, there is a problem that chlorine depletion reaction is caused using the acid formed by hydrolysis in the cured product as a catalyst to result in flowing out of chlorine ion and adverse deterioration of reliability.
On the other hand, in fabrication process of electronic products such as liquid crystal displays, personal computers, and mobile communication appliances, in the case of electrically connecting a small part such as a semiconductor device to a substrate, it is required to carry out the connection while setting fine electrodes on the opposite to each other. Further, in fabrication process of circuit in glass substrates surface, in the case where a conductive circuit is formed in the glass surface such as a lighting part of an automobile, it is required to carry out the connection while setting the glass surface and the electrode face of the conductive circuit on the opposite to each other.
As a method of connecting these electrodes, methods of connecting bumps using a solder or a conductive connection paste or directly pressure-bonding the bumps on the opposite to each other have generally been employed. Also, to protect the electrodes after the connection, methods of sealing the electrodes with resins after connection have been employed.
However, since the connection distance of fine electrodes is short, it is difficult to seal the electrodes by evenly filling the resins to the gap within a short time. Also, in the case where the glass surface and the electrode face of the conductive circuit are connected to each other, there is a problem that the connection part is heated too high if connection is carried out by soldering.
In order to solve such problems, it has been investigated to use anisotropic conductive adhesives formed to be sheet-like or paste-like adhesives by mixing conductive fine particles and insulating adhesives (e.g. reference to Japanese Patent Publication No. 3,114,162 and Japanese Kokoku Publication Hei-7-73066).
However, the conventional sheet-like anisotropic conductive adhesives have a problem that in the case where the conductive fine particles are pushed to an electrode or a bump by thermal pressure-bonding to seal the electrode, the insulating adhesives remain between the electrode and conductive fine particles to lower the connection reliability.
In the case of paste-like anisotropic conductive adhesives, the adhesives are required to have good coatability such as a high coating precision and coating efficiency at the time of applying the pastes, however there is a problem that the conventional paste-like anisotropic conductive adhesives containing a large quantity of inorganic fillers are not necessarily sufficient in fluidity and therefore not satisfactory in the coatability. In the case of producing anisotropic conductive adhesive sheets by cast method, the adhesives are also required to have good coatability. Further, there is a problem that the conductive fine particles are not evenly dispersed in the insulating adhesives, so that the conductive fine particles are agglomerated and cause short circuit of neighboring electrodes.
Therefore, inventors of the invention have previously developed a conductive connection sheet containing conductive fine particles held on an adhesive resin sheet at the time of handling the sheet and a part of the conductive fine particles are exposed out of the conductive resin sheet. The conductive connection sheet gives a high connective reliability since no insulating adhesive remains between an electrode and the conductive fine particles and the fine particles are not agglomerated (e.g. reference to Japanese Kokai Publication 2002-313143).
However, the inventors of the invention have found that the conductive connection sheet is difficult to keep both the conductive fine particles and the sheet-like shape in the case where the conductive connection sheet is used in, for example, a connection part of an electronic product or a lighting part of an automobile and is exposed in high temperature and high humidity environments, which are typically represented by a pressure cooker test (PCT), during the use.
Practically, with respect to the conductive connection sheet, there arises a problem that if the adhesive property of the sheet to the conductive fine particles is increased to improve the capability of the sheet for holding the conductive fine particles at normal temperature before curing, the sheet is softened and deteriorated in the shape-retaining capability to lower the connection reliability in high temperature and high humidity environments even after curing. On the other hand, there arises a problem that if the shape-retaining capability of the sheet in high temperature and high humidity environments is increased so as not to soften the sheet even in the high temperature and high humidity environments, the adhesive property of the sheet to the conductive fine particles at normal temperature is deteriorated to result in a problem of decrease of the capability of the sheet for holding the conductive fine particles.
The epoxy resin type curable composition may be used also as an insulating substrate material, and an insulating substrate to be used for a multilayer printed substrate and the like is required to scarcely cause effects on electric properties, to have low moisture absorbability, and to be transparent so as to make laser positioning easy and is further strongly required to have little size alteration at the time of high temperature treatment, for example, at the time of solder reflow.
The epoxy resin type curable composition may be used also as an insulating substrate material, and an insulating substrate to be used for a multilayer printed substrate and the like is required to scarcely cause effects on electric properties, to have low moisture absorbability, and to be transparent so as to make positioning of respective sheets by an optical system lens easy and is further strongly required to have little size alteration at the time of high temperature treatment, for example, at the time of solder reflow.
The properties and capabilities similar to those described above are required for die attach film insulating materials for bonding silicon chips to metal frames, multilayer boards, organic substrates such as build-up substrates, and ceramic substrates. Further, as silicon chips bearing die attach films, those to which a film is attached when they are assembled in a wafer form and those to which films are respectively attached are made available and the silicon chips, in which both cases are similarly required to have the same properties and capabilities.